Various types of sensors have been designed for measuring properties of gas. For example, sensors have been designed for measuring the percentage of a particular component in a gas mixture. Air is primarily made up of nitrogen, oxygen and lesser amounts of carbon dioxide and argon. Oxygen concentrators have been used for removing nitrogen and carbon dioxide from a flow of air. An oxygen concentrator may be used, for example, for supplying an oxygen rich product gas for medical purposes, for aircraft emergency gas supplies or for commercial purposes such as a source of oxygen for welding. The product gas from an oxygen concentrator of the molecular sieve type may be up to 95.7% oxygen with the remainder 4.3% argon which is not removed by the concentrator. It is desirable to have a gas concentration sensor capable of indicating the percentage of oxygen in the product gas from an oxygen concentrator. Also, it is desirable to have a sensor capable of indicating the bulk flow rate for the product gas from an oxygen concentrator. For medical applications, it may be critical to maintain a specific flow rate of gas having a predetermined high oxygen concentration to a patient. Sensors may be used to monitor the delivered gas to assure that the patient's requirements are met.
In a December, 1987 report for the USAF School of Aerospace Medicine, W. R. Dagle described a sensor capable of measuring both oxygen concentration of a flowing gas and the bulk flow rate. The gas flow is passed through a cylindrical chamber in a sensor housing. Two piezoelectric transducers are mounted on diametrically opposite sides of the chamber and are aligned along an axis inclined to the axis of gas flow through the chamber. A thermistor also is mounted to measure the temperature of the gas flowing through the chamber. Periodically, one of the transducers is driven with an electrical signal to emit a burst of ultrasonic energy into the chamber. The time required for the resulting ultrasonic wave to travel through the gas from the transmitting transducer to the receiving transducer is measured. The travel time is affected by the length of the path between the transducers, by the composition of the gas, by the temperature of the gas by the flow rate of the gas. Since the travel time is affected by the gas flow, the two transducers are alternately operated as transmitters and receivers so that alternate ultrasonic wave travel times will be measured in the gas flow direction and against the gas flow direction. The average of the two measured ultrasonic wave travel times through the gas and temperature of the gas are used for calculating the percentage of oxygen in the gas and the difference between the two measured times and the measured temperature are used for calculating the bulk flow rate of the gas. However, difficulties have occurred in manufacturing sensors of this type on a commercial scale which have a consistent high accuracy and a low manufacturing cost. Manufacturing tolerances are quite critical to the accuracy of the sensor.
Douglas U.S. Pat. No. 5,060,506 shows similar apparatus for measuring the concentration of a gas constituent. However, in the Douglas apparatus a very short sensing chamber (1.5 inches, 3.81 cm) is combined with a low flow rate in the sensing chamber. Together, these prevent measuring bidirectionally to produce meaningfully different travel times from which to calculate bulk flow rate. The transmitting transducer is excited with a burst of ultrasonic waves to transmit an ultrasonic wave burst through the gas. Sufficient time must be maintained between successive bursts to prevent standing waves causing noise problems in the test chamber.